1. TACAN

2. Tacan TRN 26
TACAN is an airfield navigation aid designed to give aircrew:
Bearing,
Range and
Identity Information
within approx. 100nm of the airfield.

3. Concise Details
Range approx 100 miles
Peak output power approx 450W

4. Purpose TACtical Air Navigation TACAN Slide 3 (1) & (2)
Imagine A/C takes off for an air combat sortie
After an hours dog fighting pilot needs to know where the airfield is
Draw A/C and airfield on board
What info does the pilot need to get him back to his airfield?
Where is the airfield – Direction
How far away is it – Distance
Is it his airfield – Identity

5. Purpose Magnetic north Range Bearing to beacon TACAN beacon
Pilot gets Bearing of the Beacon from the A/C
ie – the Bearing A/C has to fly to get to the Beacon
Range
Bearing to beacon
TACAN beacon

6. Bearing Principles Reflector Aerial Signal strength E S W N E S W mean
Polar diagram
Omni-directional – uniform signal strength for A/C at a fixed distance on any bearing
How can we shape polar diagram to give differing signal strength?
Reflector
Aerial
max
min
mean
E S W N E S W
Signal strength

7. Antenna Speed Revolutions per minute = 900 Revolutions per second = 15
Bearing accuracy is +/- 2°
(1) (2)
Frequency of the sine wave representing signal strength is 15Hz
It’s the phase of the 15Hz signal wrt the Main Ref Burst that represents the bearing of the A/C
It’s the number of degrees between the MRB and the 1st +ve cross-over
(3) (4)
Accuracy of ± 2° give an error of 3.5 nm at 100 miles (100 tan 2 = 3.5)
Is this error acceptable? No
How can we increase the bearing accuracy of Tacan?
Increase the number of lobes – increases the number of cross-over points
How can we increase the number of lobes?
Speed up aerial
But aerial can’t physically rotate any faster

8. Range
The Tacan distance measuring equipment (DME) employs Secondary radar techniques

9. Range
Interrogations
Replies

10. Range Time between interrogation & reply - 50μS / 12.36μS
Aircraft calculates range using the following method:
Time between interrogation & reply - 50μS / 12.36μS
This calculation provides the aircrafts slant range

11. Aircraft Interrogations
The aircraft initially interrogates the beacon at:
100 pp/sec with a pre-determined pulse spacing
This is known as Search Mode
Tacan receives and decodes the pulse pairs then transmits reply pulses

12. Aircraft Interrogations
After receiving 60% replies to its interrogations:
Aircraft interrogations reduce to 25pp/sec
This is known as Lock Mode

13. Aircraft Handling Capability
The number of aircraft in Search Mode the beacon can handle is: 27
The number of Locked On aircraft the beacon can handle is:
So knowing we have to maintain 2700pp/sec between our reference bursts, how many A/C can the beacon handle in Search Mode?
2700 ÷ 100 = 27
(1)
And the number in Lock Mode?
2700 ÷ 25 = 108
(2)
The number is realistically 100 due to the possibility of A/C interrogating the beacon in Search Mode at the same time
Approx 100

14. Identity Beacon identity produced every 37.5 seconds
Uses morse code signal
1/8 sec for a dot
3/8 sec for a dash
1/8 sec for a bit space
3/8 sec for a character space

15. Aircraft Equipment Used for military aircraft (UHF)
Aircraft has a TACVOR
VOR - VHF Omnidirectional Range navigation system

16. By the end of this lesson the student should be able to:
Objectives
By the end of this lesson the student should be able to:
State the purpose of Tacan
State the principles of operation of Tacan

17. Airfield layout

18. Any Questions?
1. What is the range of the standard Crown Watchman Radar? (A: 60Nm)
2. What Frequency Band does the Radar Operate on? (A: S Band or NATO E/F – 2 to 4GHz)
3. What is the “Uniform” PRI in µs? (A: 909 µs)
4. Why do we normally use “Staggered PRFs”? (A: To overcome the effects of Blind Speeds)
5. Our transmitter configuration is described as MOPA – What is MOPA and Describe it? (A: MOPA= Master Oscillator Power Amplifier, We use a 70MHz Oscillator and a TWT Power Amplifier).
6. What are the 2 advantageous characteristics of the TWT that causes us to use it? (A: It is a “Wideband” amplifier, that operates with a high degree of “Stability” across the frequency range)
7. What is the approximate PEAK output power of the Watchman? (A: 50 to 55kW)
8. What are the 2 pulse lengths of the Short Pulse and Long Pulse? (A: 0.4 µs and 20 µs)
9. How often are the 2 frequencies (F1 and F2) swapped in “Diversity“ mode? (A: Every 9 PRIs)
10. Why do we use 2 horns on the Watchman System? (A: We use 2 Rx beams, Main and Aux – The Aux beam just looks at the first 17Nm and is pointed slightly upwards to eliminated close-in ground clutter; The Main beam is selected for the remainder of the 60Nm range)
11. What is the approximate Azimuth Beamwidth at the 3dB points? (A: 1.5°)
12. What is the rotational Speed of the Aerial? (A: 15 rpm)
13. Give me 2 functions of the Signal Processing Rack (SPR) (A: 1. Houses Tx Driver and Receiver; 2. Carries out Signal Processing for the System; 3. Performs system timing and control functions)